In recent years, with a view point of improvement for the safety and reduction of fuel cost of automobiles, demands for increasing the strength and reducing the thickness of steel sheets used for automobiles have been increased more and more. Most of thin steel sheets used as the material sheets for automobiles are fabricated by press forming into predetermined shapes. Since the formability usually tends to be worsened as th e strength increases, various studies have been made in order to obtain steel sheets of high strength and also excellent in the formability.
Press forming includes various fabrication modes and required working characteristics vary depending on the members to be pressed in which elongation (E1) and stretch frangibility (.lambda.) are considered most important as the index for the formability required for hot rolled high strength steel sheets.
As steel sheets of excellent elongation, steel sheets that utilize TRIP (Transformation Induction Plasticity) of retained austenite have been developed as disclosed, for example, in Japanese Published Unexamined Patent Application No. Hei 7-252592, which attains 20,000 N/mm.sup.2.multidot.% or more in strength-elongation balance (TS.times.E1).
On the other hand, as steel sheets having elongation above a certain level, stretch frangibility and low yield ratio characteristics, so-called tri-phase steel sheets comprising three phase structure of ferrite-bainite-martensite have been disclosed, for example, in Japanese Published Unexamined Patent Application No. Sho 57-70257.
However, while the steel sheet of utilizing the TRIP phenomenon of the retained austenite is comparatively satisfactory in the strength-elongation balance but it shows only about 40% of the stretch frangibility (.lambda.) which does not reach a level capable of satisfying user's demand. In addition, since this steel sheet also has a relatively high yield ratio, it is poor in a shape freezing property and lacks in adaptability to the application use requiring the shape freezability.
The shape freezability means an extent of dimensional change when a work is released from a forming die upon fabrication and a material having a small amount of dimesional change is called as "material of satisfactory shape freezability". That is, when a steel sheet is manufactured by press forming into a product, it is important to ensure the dimensional accuracy required for the final product. In this case, a steel sheet pressed by a pressing machine and is in intimate contact with a forming die shows a shape conforming the forming die but changes the shape upon deforming due to the release of residual stresses. That is, any steel sheet causes considerable change of dimension upon detaching from a pressing die and it is desirable that the dimensional change is small, that is, the shape freezability is satisfactory in order to enhance the dimensional accuracy of a product. It is said that the yield strength of a material steel sheet gives most significant effect on the shape freezability and it is considered that the yield strength increases as the strength of the material steel sheet increases and, correspondingly, the shape freezability is worsened. Accordingly, it has been demanded for the development of a material having excellent shape freezability while ensuring high strength.
The .lambda. described above is a measure for expressing the stretch frangibility, which is a value determined by the following relation: EQU .lambda.=(D-D.sub.0).times.100/D.sub.0 (%)
in which D represents a hole diameter in a case where a conical punch with a 60.degree. apex is forced into an initial hole of a diameter (D.sub.0) to enlarge the hole and when a crack penetrates the sheet along the thickness.
Further, the tri-phase steel sheet is excellent in the stretch frangibility and the low yield ratio characteristic compared with a steel sheet based on the retained austenite but it has no sufficient strength-elongation balance.
As a means for obtaining a sheet having a retained austenite and still having an excellent stretch frangibility, a method of incorporating Al by about 0. 10% or more in a steel has been proposed as described, for example, in Japanese Published Unexamined Patent Application No. Hei 5-105986. However, if a great amount of Al is incorporated in the steel, the amount of oxide series inclusions increases, which results in a problem of surface defect or degradation of ductility and also gives undesired effects on the yield and the productivity.
The present invention has been accomplished taking notice on the foregoing problem and it is an object thereof to provide a hot rolled high strength steel sheet having an excellent strength-elongation balance compared with existent steel sheets and also having an excellent stretch frangibility.